/Engine/Plugins/FX/Niagara/Source/NiagaraEditor/Public/Widgets/SItemSelector.h
An optional delegate called to get a stable key (supports GetTypeHash()) for a category. If set, this delegate is used to uniquely key categories to preserve selection and expansion state when refresh is called.
WidgetArgsType & OnGetKeyForCategory &40; const FOnGetKeyForCategory & InDelegate &41;